Mesoporous ZSM-5 zeolite-supported ru nanoparticles as highly efficient catalysts for upgrading phenolic biomolecules

Article abstract of DOI:10.1021/acscatal.5b00083

Zeolite-based catalysts have been widely used in the conversion of biomass recently, but the catalytic yields of the desired products are strongly limited by the relatively small micropores of zeolite. Here, we reported a hierarchically porous ZSM-5 zeolite with micropore and b-axis-aligned mesopore-supported Ru nanoparticles (Ru/HZSM-5-OM) that are highly efficient for the hydrodeoxygenation of both small and bulky phenolic biomolecules to the corresponding alkanes. Compared with the conventional ZSM-5 zeolite-supported Ru catalyst, the high catalytic activities and alkane selectivities over Ru/HZSM-5-OM are attributed to the abundant exposed acidic sites in HZSM-5-OM with open mesopores. This feature is potentially important for future phenolic bio-oil upgrading.

Full text of DOI:10.1021/acscatal.5b00083

Research Article
pubs.acs.org/acscatalysis
Mesoporous ZSM‑5 Zeolite-Supported Ru Nanoparticles as Highly
Efficient Catalysts for Upgrading Phenolic Biomolecules
Liang Wang,*,† Jian Zhang,† Xianfeng Yi,‡ Anmin Zheng,*,‡ Feng Deng,‡ Chunyu Chen,† Yanyan Ji,†
Fujian Liu,† Xiangju Meng,† and Feng-Shou Xiao*,†
†Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310028, China
‡National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular
Physics and Mathematics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Science, Wuhan 430071, China
S
* Supporting Information
ABSTRACT: Zeolite-based catalysts have been widely used in
the conversion of biomass recently, but the catalytic yields
of the desired products are strongly limited by the relatively
small micropores of zeolite. Here, we reported a hierarchically
porous ZSM-5 zeolite with micropore and b-axis-aligned
mesopore-supported Ru nanoparticles (Ru/HZSM-5-OM)
that are highly efficient for the hydrodeoxygenation of both
small and bulky phenolic biomolecules to the corresponding
alkanes. Compared with the conventional ZSM-5 zeolite-
supported Ru catalyst, the high catalytic activities and alkane
selectivities over Ru/HZSM-5-OM are attributed to the abun-
dant exposed acidic sites in HZSM-5-OM with open mesopores. This feature is potentially important for future phenolic bio-oil
upgrading.
KEYWORDS: phenolic biomolecules, hydrodeoxygenation, mesoporous zeolite, Ru catalyst, biomass conversion
microporosity of the zeolite-based catalysts strongly limits the
conversion of bulky biomolecules. Therefore, the production
of relatively bulky bioalkanes is a challenge in the hydro-
deoxygenation of phenolic bio-oil over zeolite-based catalysts.
Zeolites with hierarchical porous structures containing both
micro- and mesopores, which are designated as mesoporous
zeolites, have been developed.34−47 The mesoporous zeolite-
based catalysts have exhibited unique catalytic properties in
many reactions compared with the conventional zeolites. For
example, mesoporous ZSM-5 and Y zeolite-supported Pd and
Pt−Pd metals are more active in hydrodesulfurization reactions
than conventional zeolite-based catalysts.45,46 The ZSM-5-
based catalysts with mesopores gave much higher selectivity to
C5−C11 isoparaffins in Fischer−Tropsch reactions than the
conventional ZSM-5-based catalyst.47 However, catalytic con-
version of biomass over the mesoporous zeolite-based catalysts
is rarely reported, although the mesopores are favorable for the
conversion of bulky biomass molecules. Herein, we show that
the mesoporous ZSM-5 zeolite (ZSM-5-M)-supported Ru
nanoparticles are efficient catalysts for the hydrodeoxygenation
of the phenolic compounds in bio-oil to alkanes. Particularly,
b-axis-aligned mesoporous ZSM-5 (ZSM-5-OM), a new type of
mesoporous zeolite with b-axis-aligned mesopore-supported
1. INTRODUCTION
Biomass, which is regarded as a renewable feedstock, has attracted
attention for the production of fine chemicals and fuels.1−15
Many economically viable processes have been developed for
the conversion of biomass.1−5 For example, fast pyrolysis of
lignocellulosic biomass to produce phenolic bio-oil has been
deemed a promising alternative to fossil fuels.16−24 However,
the direct use of phenolic bio-oils is impossible because their
high oxygen content leads to low energy density, high viscosity,
and low stability.16−19 Therefore, upgrading bio-oils by hydro-
deoxygenation is important to produce high-quality alkane bio-oils.
Recently, many Pd-, Ni-, and Pt-based catalysts have been re-
ported to effectively catalyze the hydrodeoxygenation of the
major components of phenolic bio-oils, such as phenol, syringol,
furfural, and their derivatives.25−30 To obtain high-quality
alkanes, acids (e.g., phosphoric acid, acidic ionic liquids) mixed
with metals are generally required for the cleavage of C−O
bonds, which is a key step in the hydrodeoxygenation process.
However, these homogeneous acids add complexity when
regenerating the reaction systems, which is energy-consuming.
More recently, as typical solid acids, aluminosilicate zeolites
(e.g., ZSM-5, Beta) combined with metals have been employed
to catalyze the hydrodeoxygenation of phenolic compounds to
alkanes.7,25,26,31−33 Because of the obvious advantages of high
stability, easy separation, and abundant acidic sites, zeolite-
based catalysts are regarded to be one of the most promising
catalysts in the application of bio-oil upgrading. However, the
Received: October 31, 2014
Revised: March 17, 2015
© XXXX American Chemical Society
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Ru nanoparticles, are very active for the production of alkane
bio-oils because of the open mesopores on ZSM-5-OM with
rich exposed acid sites for catalyzing the cleavage of C−O
bonds, a key step in the hydrodeoxygenation of phenolic
molecules to alkanes.
added to 50 mL of RuCl3 solution with the desired Ru
concentration. After adding urea (molar ratio of urea/Ru of
100) and stirring at 90 °C for 4 h in a closed reactor kept away
from light, the solid sample was filtrated, washed with a large
amount of water, dried at 100 °C for 12 h, calcined at 400 °C
for 3 h, and reduced by H2 at 250 °C for 2 h to obtain the final
samples.
2. EXPERIMENTAL SECTION
2.6. Characterization. Powder X-ray diffraction patterns
(XRD) were obtained with a Rigaku powder X-ray diffractom-
eter using CuKα radiation (λ = 0.1542 nm). X-ray photo-
electron spectroscopy (XPS) was performed on a Thermo
ESCALAB 250 with Al Kα radiation at θ = 90° for the X-ray
source. The binding energies were calibrated using the C 1s
peak at 285.0 eV. The metal content was determined by induc-
tively coupled plasma spectroscopy (ICP, PerkinElmer 3300DV).
Nitrogen sorption isotherms were measured at −196 °C using a
Micromeritics ASAP 2020 M system. The samples were degassed
for 10 h at 150 °C before the measurements. The mesopore
size distributions were determined using the Barrett−Joyner−
Halenda (BJH) model. Scanning electron microscopy (SEM)
was performed using a Hitachi SU 1510. Transmission electron
microscope (TEM) images were collected using a Hitachi
HT-7700. NMR experiments were performed on a Bruker
Ascend-500 spectrometer at a resonance frequency of 202.63
MHz for 31P, with a 4 mm triple-resonance MAS probe at a
spinning rate of 10 kHz. 31P MAS NMR spectra with high
power proton decoupling were recorded using a π/2 pulse
length of 4.1 μs and a recycle delay of 30 s. The chemical shift
of 31P was referenced to 1 M aqueous H3PO4. Prior to the ad-
sorption of the probe molecules, the acid samples were placed
in glass tubes and connected to a vacuum line for dehydra-
tion. The temperature was gradually increased at a rate of
1 °C min−1, and the samples were kept at a final temperature of
400 °C and a pressure below 10−3 Pa over a period of 10 h and
were then cooled. After the samples cooled to ambient tem-
perature, a known amount of trimethylphosphine oxide (TMPO)
was introduced into the acid. The activated samples were frozen
by liquid N2, followed by elimination of the physisorbed probe
molecules by evacuation at room temperature for 10 min. Finally,
the sample tubes were flame-sealed. The preparation of the
TMPO adsorbed sample was performed according to the
method proposed by our previous work.49 Prior to the NMR
experiments, the sealed samples were transferred into ZrO2
rotors with a Kel-F end-cap under a dry nitrogen atmosphere in
a glovebox.
2.7. Catalytic Tests. The hydrodeoxygenation was
performed in a high-pressure autoclave with a magnetic stirrer
(1200 rpm). Typically, the substrate, catalyst, and solvent were
mixed in the reactor by stirring for 10 min at room tempera-
ture. Then, hydrogen was introduced and maintained at the
desired pressure, and the reaction system was heated to a given
temperature (the temperature was measured with a thermom-
eter in an oil bath, and the hydrogen pressure in the autoclave
was recorded at the reaction temperature). After the reaction,
the product was removed from the reaction system and ana-
lyzed by gas chromatography (GC-14C, Shimadzu, using a
flame ionization detector) with a flexible quartz capillary
column coated with OV-17 and FFAP and a high-pressure
liquid chromatograph (HPLC1200, Agilent) equipped with a
Ca cation exchange column and methanol (0.4 mL/min) as the
eluent. The substrate conversions and product selectivities are
based on the carbon atoms in the molecules using ethylbenzene
as the internal standard. The recyclability of the catalyst was
2.1. Materials. All reagents were of analytical grade and
were used as purchased without further purification. Tetra-
propyl ammonium hydroxide (TPAOH, 20.1 wt %), CH3I,
DMF, phenol, 5-hydroxymethylfurfural (HMF), glucose,
1-ethyl-3-methylimidazolium chloride, and 2,6-dimethoxyphenol
were purchased from Aladdin Company. 4-Allyl-2-methoxyphenol,
4-allyl-2,6-dimethoxyphenol, 4-hydroxy-3-methoxybenzaldehyde,
4-ethyl-2-methoxyphenol, and biphenyl-2,2′-diol, 1-(4-hydroxy-
phenyl)-2-phenylethanone were purchased from Kaide Chemical
Co. Tetraethyl orthosilicate (TEOS), NaAlO2, petroleum ether,
ammonium nitrate, and NaOH were purchased from Tianjin
Guangfu Chemical Co. Cationic polydiallyldimethylammonium
chloride (PDADMAC, 10 wt %, molecular weight approximately
1.5 × 105) was purchased from Yinhu Chemical Co. RuCl3 was
obtained from Huishui Tech. Co. Copolymer polystyrene-co-4-
polyvinylpyridine (molecular weight approximately 1.6 × 105, PSt-
co-P4VP) was obtained from Sigma-Aldrich Company. Quaternary
ammoniation of PSt-co-P4VP by CH3I to obtain C-PSt-co-P4VP
was performed according to the literature.44
2.2. Synthesis of ZSM-5 and ZSM-5-M. In a typical
synthesis of ZSM-5 zeolite, (1) 8 mL of tetrapropyl ammonium
hydroxide (TAPOH, 19.4 wt %) and 93 mg of NaAlO2 were
added to 20 mL of water. (2) Then, the mixture was stirred at
room temperature for 2 h, and 7 mL of TEOS was added. (3)
After stirring overnight, the gel was transferred into an auto-
clave to crystallize at 180 °C for 3 days. (4) By filtrating, drying,
and calcining at 550 °C for 4 h, the ZSM-5 sample was ob-
tained. The ZSM-5-M was synthesized under the same condi-
tions except for the addition of 1.2 g of PDADMAC after the addi-
tion of TEOS. After ammonium anion exchange with ammonium
nitrate and calcination, the H-form zeolite was obtained.
2.3. Synthesis of ZSM-5-OM. In a typical run, 0.08 g of
NaAlO2, 12.5 mL of TPAOH (20.1 wt %), and 7.0 mL of
TEOS were mixed, followed by the addition of 20 mL of water.
After stirring at room temperature for 6 h and aging at 100 °C
for 2 h, a clear zeolite precursor was obtained. Meanwhile, 2 g
of quaternary ammoniated polyvinylpyridine polymer (molec-
ular weight approximately 1.6 × 105, C-PSt-co-P4VP) was
dissolved into 5 mL of water, followed by the addition of 3 mL
of TPAOH, giving a C-PSt-co-P4VP solution. After mixing the
clear zeolite precursor with the C-PSt-co-P4VP solution, the
mixture was stirred for 24 h at room temperature, then trans-
ferred into an autoclave for crystallization at 180 °C for 3.5 days.
After filtration, drying, calcination at 550 °C in oxygen, ammonium
anion exchange with ammonium nitrate, and calcination again, the
H-form zeolite was obtained.
2.4. Synthesis of Ru/HZSM-5, Ru/HZSM-5-M, Ru/HZSM-
5-OM, Pt/HZSM-5, Pd/HZSM-5, and Ru/NaZSM-5. In a
typical run, 1 g of the solid support (e.g., HZSM-5) was stirred
in 50 mL of RuCl3 solution with the desired Ru concentration for
12 h, followed by evaporating the excess water at 80 °C, heating at
100 °C for 12 h, and washing with a large amount of water. Then,
the solid powder was calcined in air at 400 °C for 2 h and was
reduced by H2 at 250 °C for 2 h to obtain the final samples.
2.5. Synthesis of Ru/SBA-15, Ru/Al2O3, and Au/ZSM-5.
In a typical run, 1 g of the solid support (e.g., SBA-15) was
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10 min (the temperature was measured with a thermometer
in the oil bath). Then, the catalyst was added to the reaction
mixture. After the reaction, the HMF product was analyzed by
gas chromatography (GC-14C) with a flexible quartz capillary
column (OV-17) using ethylbenzene as an internal standard.
3. RESULT AND DISCUSSION
3.1. Synthesis and Characterization. The ZSM-5-OM
sample (Si/Al = 42) was synthesized according to the reported
literature, using tetrapropyl ammonium hydroxide and
copolymer polystyrene-co-4-polyvinylpridine as templates.44
After calcination to remove the organic template and ion-
exchange treatment, the H-form ZSM-5-OM zeolite (HZSM-5-
OM) was obtained (Figure S1−S3). Then, the Ru nano-
particles were loaded on the HZSM-5-OM support by the
impregnation method (Ru/HZSM-5-OM, Figure S4−S8). The
Ru content in Ru/HZSM-5-OM was 1.1 wt %, as analyzed by
inductively coupled plasma (ICP). For comparison, the
conventional ZSM-5 zeolite (Ru/HZSM-5) and mesoporous
ZSM-5 zeolite-supported Ru catalyst (Ru/HZSM-5-M) was
also prepared (Figure S4−S8). The textural parameters of
various samples are listed in Table S1.
Figure 1. 31P NMR spectra of TMPO adsorbed on (a) HZSM-5, (b)
HZSM-5-M, and (c) HZSM-5-OM. The asterisks in the 31P NMR
spectra denote spinning sidebands due to the strong interactions
between TMPO and the Brønsted acid sites.
Figure 1 shows the 31P NMR spectra of trimethylphosphine
oxide (TMPO) adsorbed on various samples, which is a unique
and practical technique to characterize the acid strength and
host/guest interaction in solid acid catalysts.48,49 On the basis
of our previous work, the confinement effect of the zeolite
framework is important in determining the 31P chemical shift of
TMPO adsorbed on the Brønsted acid sites.49,50 The channel
diameter of ZSM-5 is approximately 0.55−0.60 nm, which is
comparable to the molecular size of TMPO with a kinetic
diameter of 0.55 nm. The more complete pore structure inside
ZSM-5 facilitates the investigation of the long-range electro-
static effects from the Madelung potential of the zeolite frame-
work (confinement effect), which is pronounced for the micro-
porous channels of ZSM-5 zeolite.50 Thus, the 31P chemical
shift can be used to characterize the integrity of the zeolite
micropores. As shown in Figure 1, the HZSM-5, HZSM-5-M,
and HZSM-5-OM samples give five signals at 51−53, 65, 70,
76, and 82−86 ppm, associated with TMPO adsorbed at the
Table 1. Measurement Data for the 31P NMR of TMPO
Adsorbed on HZSM-5, HZSM-5-M, and HZSM-5-OM
31P NMR peak/ppm relative concentration (%)
samples
51−53
65
70
76
82−86
H-ZSM-5
16.4
25.0
36.5
16.5
18.0
19.3
3.4
2.7
3.1
49.5
44.5
37.1
14.3
9.9
H-ZSM-5-M
H-ZSM-5-OM
4.1
tested by separating it from the reaction system by centri-
fugation, washing it with a large quantity of ethanol and drying
it at 100 °C for 4 h.
The conversion of glucose to 5-hydroxymethylfurfural
(HMF) was performed in a 25 mL glass reactor with a mag-
netic stirrer in an oil bath. In a typical run, glucose and [Emim]
Cl (1-ethyl-3-methylimidazolium chloride) were mixed in the
reactor and stirred (1200 rpm) at the reaction temperature for
a
Table 2. Catalytic Data for the Hydrodeoxygenation of Phenol and 2,6-Dimethoxyphenol over Various Catalysts
b
product selectivity (%)
entry
substrate
phenol
catalyst
conv (%)
cyclohexane
cyclohexanol
cyclohexanone
carbon balance (%)
1
2
3
4
5
6
7
8
9
10
Ru/HZSM-5
Pt/HZSM-5
Pd/HZSM-5
Ni/HZSM-5
Au/HZSM-5
Ru/NaZSM-5
Ru/Al2O3
>99.5
>99.5
>99.5
42.2
91.0
55.0
88.4
40.0
2.0
7.0
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
phenol
phenol
phenol
phenol
phenol
phenol
phenol
phenol
phenol
29.0
11.6
15.5
16.0
44.5
<1.0
98.1
99.0
90.0
94.0
4.0
1.0
10.0
6.0
0.6
0.9
>99.5
89.9
Ru/SBA-15
Ru/HZSM-5-M
Ru/HZSM-5-OM
>99.5
>99.5
95.4
95.0
5.0
c
cyclohexane
55.4
methanol
17.0
16.9
19.7
others
d
11
12
13
2,6-dimethoxyphenol
2,6-dimethoxyphenol
2,6-dimethoxyphenol
Ru/HZSM-5
70.0
81.0
97.5
15.0
6.9
93.2
d
Ru/HZSM-5-M
Ru/HZSM-5-OM
57.7
91.0
d
70.0
2.0
92.4
a
b
Reaction conditions: 150 °C, 4 h, 4.0 MPa of H2, 50 mg of catalyst, 1 mmol of phenolic substrate, 8 mL of water. (C atoms in each product/total
c
d
C atoms in all products) × 100%. Cyclohexanol- and cyclohexanone-derived molecules and some others. The undetected carbon was mainly due
to the volatilization of the methanol product.
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different acid sites on the varied zeolites. Notably, the 31P
chemical shift is at 86 ppm in HZSM-5 and is attributed to the
Al12−O24−Si12 site in ZSM-5 zeolite with a 10-membered ring
(10MR) structure49 was shifted slightly to 85 ppm over the
HZSM-5-M and significantly to 82 ppm over HZSM-OM. This
significant 31P chemical shift toward the high-field indicates that
the host/guest interactions are decreased by the presence of
open mesopores in H-ZSM-5-OM. Furthermore, the quantita-
tive amount of each acid site can be obtained from the 31P peak
intensity. As listed in Table 1, the relative amount of 31P peak at
82−86 ppm is dramatically decreased from 14.3% (HZSM-5)
to 9.9% (HZSM-5-M) and 4.1% (HZSM-5-OM). Meanwhile,
the 31P peak at approximately 50 ppm, assigned to TMPO
adsorbed in the mesopores and/or on the external acid sites,
significantly increased in intensity from 16.4% (HZSM-5) to
25.0% (HZSM-5-M) and 36.5% (HZSM-5-OM), indicating
that the HZSM-5-OM has significantly more exposed acidic
sites in the open mesopores. This feature is favorable for the
catalytic conversion of bulky molecules, such as bulky biomass
molecules. In addition, these samples exhibited similar peaks at
76, 70, and 65 ppm, which are ascribed to TMPO adsorbed on
the acidic protons located at different T sites with different acid
strengths in H-ZSM-5 zeolite.50
3.2. Hydrodeoxygenation of Phenol. Table 2 shows the
catalytic data for the hydrodeoxygenation of the relatively small
molecule of phenol and the bulky molecule of 2,6-dimethoxyphenol
Figure 2. Dependences of phenol conversion and product selectivity
on time over (A) Ru/HZSM-5 and (B) Ru/HZSM-5-OM catalysts.
The reaction conditions are the same to those in Table 2.
Figure 3. Dependences of substrate conversion on time over Ru/HZSM-5, Ru/HZSM-5-M, and Ru/HZSM-5-OM catalysts in (A) hydrogenation of
phenol, (B) hydrogenation of cyclohexanone, (C) dehydration of cyclohexanol, (D) hydrogenation of cyclohexene. Reaction conditions: 150 °C, 4.0
MP of H2 (N2 was used in the dehydration reaction), 30 mg of catalyst, 1 mmol of phenolic substrate, 8 mL of water. The hydrogen was introduced
into the reactor after the required temperature was achieved.
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Table 3. Reaction Rates of Various Catalysts in Different Reactions
reaction rate (mmol g−1 h−1
Ru/HZSM-5-M
)
entry
reactions
Ru/HZSM-5
Ru/HZSM-5-OM
1
2
3
4
5
6
phenol hydrogenation
8.5
13.3
28.0
33.9
6.9
8.8
14.0
31.8
31.5
7.2
6.2
10.7
34.6
29.7
6.0
cyclohexanone hydrogenation
cyclohexanol dehydration
cyclohexene hydrogenation
2,6-dimethoxyphenol hydrogenation
2-methoxycyclohexanol dehydration
0.4
2.0
4.4
as the model phenolic molecules of bio-oils. Typically, in the
hydrodeoxygenation of phenol to cyclohexane, cyclohexanone,
cyclohexanol, and cyclohexene are formed as intermediates
(Scheme S1, Figure 2). The Ru/HZSM-5, Pd/HZSM-5, and
Pt/HZSM-5 catalysts are catalytically active, with phenol
conversion higher than 99.5% (entries 1−3 in Table 2). In
addition, Ru/HZSM-5 shows higher cyclohexane selectivity
(91.0%) than Pt/HZSM-5 and Pd/HZSM-5 (55.0−88.4%).
These results suggest that Ru/HZSM-5 is suitable for the
hydrodeoxygenation of phenol.
The Na-form of ZSM-5 zeolite-supported Ru nanoparticles
(Ru/NaZSM-5) gave cyclohexanol as a major product with
a selectivity of 99.0% (entry 6 in Table 2). Similarly, the
Ru/Al2O3 and Ru/SBA-15 catalysts have cyclohexanol as a
major product (selectivities at 90.0−94.0%, entries 7 and 8 in
Table 2). This phenomenon is attributed to the absence of
Brønsted acid sites on NaZSM-5, Al2O3, and SBA-15 supports,
which are inactive for the dehydration of cyclohexanol, an
important step during the hydrodeoxygenation of phenol to
cyclohexane (Scheme S1). Ru/HZSM-5, Ru/HZSM-5-M, and
Ru/HZSM-5-OM catalysts have similar conversion (>99.5%)
and cyclohexane selectivities (91.0−95.4%) in phenol hydro-
deoxygenation (entries 1, 9, and 10 in Table 2). To understand
their activities, Figure 3 shows the dependences of substrate
conversion on time over these catalysts in the elementary reac-
tions of phenol dehydroxygenation, including hydrogenation of
phenol, hydrogenation of cyclohexanone, dehydration of cyclo-
hexanol, and hydrogenation of cyclohexene (Scheme S1).
Based on the fitted straight line, the reaction rates of each
catalyst are calculated and are presented in Table 3.
Figure 4. Photographs of the reaction mixture of 2,6-dimethoxyphenol
in water (A) before and (B) after the 5-g scale hydrodeoxygenation
over Ru/HZSM-5-OM catalyst. (C) Enlarged photograph showing the
phase separation of cyclohexane from water.
The reaction rates are in the following: phenol hydrogenation
< cyclohexanone hydrogenation < cyclohexanol dehydration <
cyclohexene hydrogenation. These results indicate that the
hydrogenation of phenol is a control step for the overall reac-
tion rate of phenol hydrodeoxygenation. Because Ru/HZSM-5,
Ru/HZSM-5-M, and Ru/HZSM-5-OM catalysts have similar
reaction rates in phenol hydrogenation (6.2−8.8 mmol g−1 h−1,
Table 3), it is reasonable that the three catalysts have similar
conversion and selectivity in phenol hydrodeoxygenation.
3.3. Hydrodeoxygenation of 2,6-Dimethoxyphenol.
Ru/HZSM-5-M, Ru/HZSM-5-OM, and Ru/HZSM-5 catalysts
exhibit distinguishable catalytic performance in the hydro-
deoxygenation of bulky 2,6-dimethoxyphenol (Tables 2 and S2).
For example, Ru/HZSM-5-OM has a cyclohexane selectivity of
70.0% (entry 13 in Table 2), whereas Ru/HZSM-5 has a
selectivity of 55.4% (entry 11 in Table 2). In this reaction, the
dehydration step (dehydration of 2-methoxycyclohexanol) is
the control step, giving reaction rates of 0.4−4.4 mmol g−1 h−1
(Table 3). Therefore, the high yield of cyclohexane product
in the hydrodeoxygenation of 2,6-dimethoxyphenol over
Ru/HZSM-5-OM is attributed to its higher rate (4.4 mmol
g−1 h−1) in the 2-methoxycyclohexanol dehydration step than
Figure 5. Recycling tests of Ru/HZSM-5-OM in the hydrodeoxygenation
of 2,6-dimethoxyphenol. The reaction conditions are the same as those in
Table 1. The catalyst was calcined at 480 °C for 4 h before the 8th use.
that over Ru/HZSM-5 and Ru/HZSM-5-M (0.4−2.0 mmol
g−1 h−1).
Because the bulky 2,6-dimethoxyphenol molecule could not
transfer through the small micropores of ZSM-5 zeolite, the
dehydration of 2,6-dimethoxyphenol occurs on the external/
mesopore acid sites of the ZSM-5-based catalysts.35−46 There-
fore, the different activities of the Ru/HZSM-5, Ru/HZSM-5-
M, and Ru/HZSM-5-OM catalysts in the 2,6-dimethoxyphenol
dehydration are attributed to the differences in the accessible
acidic sites of 2,6-dimethoxyphenol on the zeolite catalysts.
The Ru/HZSM-5-OM has more accessible acidic sites in the
mesopores for the 2,6-dimethoxyphenol molecules than
Ru/HZSM-5, in good agreement with the observations from
the 31P MAS NMR technique (Figures 1, S9, and S10, Tables 1,
S1, S3, and S4).
Based on these observations, it is proposed that the sig-
nificantly better catalytic properties in the hydrodeoxygenation
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a
Table 4. Catalytic Hydrodeoxygenation of Various Phenolic Monomers and Dimmers over Various Catalysts
a
b
Reaction conditions: 170 °C, 4 h, 4.0 MP of H2, 50 mg of catalyst, 1 mmol of phenolic substrate, 8 mL of water. The undetected carbon was
c
d
mainly due to the volatilization of methanol product. The data were obtained from ref 7. Cyclic alcohols, ketones, aromatics, and some others.
of 2,6-dimethoxyphenol over Ru/HZSM-5-OM than over
Ru/HZSM-5-M and Ru/HZSM-5 result from the presence of
b-axis-aligned mesopores in the ZSM-5 crystals, which improve
the access of bulky molecules to the acidic sites in the zeolite
catalysts.
A series of solvents, such as water, methanol, ethanol, ethyl
acetate, and dodecane, were employed in the dehydration
of phenol, and water was a suitable solvent in this reaction
(Table S5). After hydrodeoxygenation in water solvent, the
water-soluble phenolic molecules were transformed into the
water insoluble alkanes, where the phase separation of alkanes
from the water solvent promoted the shift of the reaction
balance to the formation of alkanes (Figure 4). Because the
production of phenolic molecules from biomass is always
performed with water, the use of water solvent in this reaction
has advantages, such as high efficiency, low cost, and easy
separation of the cyclohexane product.51 Furthermore, we
performed the 2,6-dimethoxyphenol hydrodeoxygenation on a
5-g scale in water solvent over Ru/HZSM-5-OM catalyst. In
this case, high 2,6-dimethoxyphenol conversion (94.4%) and
good cyclohexane selectivity (71.4%) were obtained. The phase
separation of the cyclohexane product from water solvent was
obvious (Figure 4C).
Figure 5 shows the recycling tests of Ru/HZSM-5-OM
catalyst in the hydrodeoxygenation of 2,6-dimethoxyphenol.
After recycling ten times, Ru/HZSM-5-OM still gives high
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Research Article
2,6-dimethoxyphenol conversion (95.8%) and good cyclo-
hexane selectivity (69.0%), which are similar to those (97.5 and
70.0%) of the as-synthesized Ru/HZSM-5-OM. These results
confirm the excellent recyclability of the Ru/HZSM-5-OM
catalyst. At the same time, the Ru species in the liquid collected
from the reaction mixture is undetectable by ICP analysis,
indicating that metal leaching is almost negligible under the
reaction conditions.
ASSOCIATED CONTENT
■
S
* Supporting Information
The following file is available free of charge on the ACS
Publications website at DOI: 10.1021/acscatal.5b00083.
TEM, XPS, XRD, and SEM characterizations and catalytic
data (PDF)
3.4. Hydrodeoxygenation of Various Phenolic Mono-
mers and Dimers. Table 4 shows the catalytic data for the Ru
catalysts in the hydrodeoxygenation of various biofuel-derived
phenolic monomers and dimers to produce C6−C14 alkanes.
The various Ru catalysts are active with alkanes, aromatics,
cycloalcohols, cycloketones, and methanol as products at
reaction temperature of 170 °C. For example, in the hydro-
genation of 1a, Ru/HZSM-5-OM gives a full conversion of 1a
with selectivity to 1b at 83.0% (entry 1 in Table 4). In contrast,
Ru/HZSM-5 exhibits lower 1a conversion (77.4%) and 1b
selectivity (49.2%, entry 3 in Table 4). The combined Pd/C
and phosphoric acid catalyst, one of the most active catalysts for
hydrodeoxygenation reported previously, exhibits 1a conver-
sion at 99.0% and 1b selectivity at 65.0% at relatively high
temperature (250 °C, entry 4 in Table 4).7
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: liangwang@zju.edu.cn (L.W.).
*E-mail: zhenganm@wipm.ac.cn (A.Z.).
*E-mail: fsxiao@zju.edu.cn (F.-S.X.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (21333009, 21403192, and 21403193),
the National High-Tech Research and Development program
of China (2013AA065301), and the China Postdoctoral
Science Foundation (2014M550318).
Furthermore, Ru/HZSM-5-OM has much higher activity and
C9 alkane selectivity in the hydrodeoxygenation of 2a than
Ru/HZSM-5-M and Ru/HZSM-5 catalysts. Particularly,
Ru/HZSM-5-OM has higher activity and selectivity than the
combined Pd/C and phosphoric acid catalyst (entries 5 and 8
in Table 4). When substrates 3a and 4a are used, Ru/HZSM-5-
OM is also very active (entries 9 and 10 in Table 4). When
5-hydroxymethylfurfural (HMF, 5a) is chosen as a substrate,
Ru/HZSM-5-OM has much higher product selectivity than
Ru/HZSM-5-M and Ru/HZSM-5 (entries 11−13 in Table 4).
In addition to the phenolic monomers, we also compared the
catalytic performance in the hydrodeoxygenation of phenolic
dimers of 6a and 7a over the catalysts. Ru/HZSM-5-OM shows
significantly higher C12 and C14 alkane selectivities for 6b and
7b (81.4 and 62.4%, entries 14 and 17 in Table 4) than
Ru/HZSM-5-M (52.2 and 37.7%, entries 15 and 18 in Table 4)
and Ru/HZSM-5 (44.7 and 12.4%, entries 16 and 19 in Table 4).
These results demonstrate the excellent catalytic properties of
Ru/HZSM-5-OM in the hydrodeoxygenation of bio-oil molecules.
3.5. Conversion of Glucose to HMF. It is interesting to
note that the concept of mesopore zeolite-based catalysts for
biomass conversion is not limited to hydrodeoxygenation but
can be applied to the production of platform chemicals, such as
HMF. For example, in the conversion of glucose to the
platform molecule HMF, HZSM-5-OM exhibits more excellent
catalytic properties than the HZSM-5-M and HZSM-5 catalysts
(Figure S11). The further details are still under investigation.
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